An inherent requirement with a portable hand-held device such as a mobile phone is the inclusion of a battery of sufficient performance to achieve the necessary electrical power supply for roving operation of that portable hand-held device. An example of such a battery is a lithium ion battery. Lithium ion batteries are popular for handheld devices due to their high energy density. However, in order to reach their full capacity it is necessary to charge them up to a max voltage, typically 4.2V. If charged only up to say 4.0V the battery would only hold about 75% of its maximum capacity. However charging a lithium ion battery to higher that the max. voltage (4.2V) can be dangerous as this can result into overheating, swelling and short-circuiting of the battery.
Clearly, it is necessary to regularly re-charge the battery as fully but also as safely as possible. With hand-held devices it also is possible that the device is continued to be used whilst the charging operation is performed. In such circumstances while the battery is being charged it may also be providing a relatively high load current for device operation. This can create problems with respect to ensuring that the particular defined target voltage necessary in order to achieve a fully charged state can be maintained.
Previously, in order to attempt to achieve full charging, measurement of the non or minimal loaded open circuit battery voltage (OCV) has been achieved in order to adjust the defined target voltage necessary for a fully charged state. For practical and safety reasons the battery voltage is not measured directly at the battery tabs, but at a point in the power supply circuit where there would be some wiring and typically a safety circuit between the measuring point and the battery tabs. Unfortunately, the impedance between the battery terminals and the measuring point together with the load current will result in a voltage drop and therefore a lower measured open circuit battery voltage in comparison with the actual open circuit voltage for that battery. This difference between the measured and the actual open circuit voltage can be calculated using Ohm's law. In any event, it will be understood that a high load generally results in a high deviation between the measured open circuit voltage and the actual open circuit voltage for the battery. In such circumstances, previous battery charging arrangements generally ensure that full charging of the battery is postponed until the battery load current is low, that is to say the portable hand held device was not in operation. This may be inconvenient for the user.